As the demand for increased bandwidth in wireless communications systems continues to grow, designers of the wireless communications systems are bumping into limitations (both physical and regulatory) on the amount of spectrum that can be used. With more wireless communications systems being put into service, the amount of spectrum that each can use is naturally reduced. Additional restrictions may be placed by governmental bodies, such as the Federal Communications Commission (FCC) in the United States. The FCC restricts the use of wireless communications systems to approved bands.
With more wireless communications devices sharing a fixed amount of spectrum, interference can play a significant role in the performance of a wireless communications system. One technique that can help a wireless communications system achieve improved performance in the presence of interference entails the use of multiple transmit antennas. This technique is commonly referred to as transmit antenna diversity. Each of the transmit antennas can transmit either a portion of the overall signal or the same signal that has been encoded differently. At the receive end, one or more receive antennas can be used to receive the signal. With processing, the original transmitted signal can be extracted from what was received.
There are two main types of transmit antenna diversity. A first, referred to as open-loop transmit diversity (OLTD), simply uses multiple transmit antenna and it is up to the receiver to decode the transmitted information. A second, referred to as closed-loop transmit diversity (CLTD), uses a feedback loop to provide information regarding the transmission channel in an attempt to improve the overall performance of the wireless communications system.
Currently used CLTD systems, such as those in the downlink (from base station to mobile station) of a WCDMA (wideband CDMA, a third generation mobile communications standard) compliant wireless communications network, can provide significant performance gains over OLTD systems. In these systems, the transmitter is provided channel state information (CSI) by the receiver. When the system is using frequency division duplex (FDD), the CSI can be provided by a feedback channel and when the system is using time division duplex (TDD), CSI can be determined by examining an uplink channel.
One disadvantage of the prior art is that they assume that multipath interference is negligible and therefore simple maximum ratio combining is sufficient to combine the signals from the various transmit antennas. For systems with a large spreading gain, this assumption may be valid. However, for high-data rate systems that use a small spreading gain, multipath interference may manifest itself as inter-symbol interference (ISI) and/or multi-user interference (MUI). The ISI and/or MUI can affect the overall performance of the CLTD system.